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|A Dozen Fun Facts about Contact Angle|
Our feature article in last month's
newsletter detailing a dozen fun facts about surface tension was
such a big hit (based on the feedback we got) that we've decided to
share a dozen fun facts about contact angle in this month's edition.
1. Increased roughness will decrease the measured contact angle on hydrophilic surfaces but increase the measured contact angle on hydrophobic surfaces. In other words, roughness amplifies both hydrophobicity and hydrophilicity making high contact angles higher and low contact angles lower.1
2. Generally, as temperature increases, contact angle decreases.2 The surface tension of most liquids will decreased as temperature is increased due to an increase in molecular thermal activity. Thus wetting improves.
3. If the left and right contact angle are not close to the same value and the sample is not tilted, this would generally indicate that the surface is not so homogenous in terms of chemical and structural composition. That is, the wetting properties at the spot where the left contact angle forms is different from the wetting properties where the right contact angle forms. Unsymmetrical sessile drops can also indicate non-uniform surface contamination.
4. If the contact angle hysteresis is small, this generally indicates that the surface is very homogeneous with regard to both chemical composition as well as structure.3
5. Wetting behaviors described by the Young equation apply to ideal surfaces.4 Most surfaces are not ideal.
6. Even ideal surfaces exhibit contact angle hysteresis. This is explained by pinning of the three-phase line. The intermolecular forces between the liquid and the solid (e.g., a silicon wafer) explain the pinning behavior which results in a range of possible contact angles even when there is no roughness. Measuring the change in water contact angle of a sessile drop as it evaporates on a wafer is one way to observe and measure this pinning force and contact angle hysteresis.
7. Drop volume shouldn't matter very much (especially on ideal and non-rough surfaces). That is, the contact angle of a 1 μL drop should measure about the same as a 5 μL drop. However, since most surfaces are less than ideal, the variation in contact angle between drops of different volumes on the same surface may be greater than zero.
8. Drop volume does matter for advancing and receding contact angles using the tilting plate method. As volume increases so too do the effects of gravity. Larger drops roll off sooner than smaller drops.
9. The most common liquid used for contact angle testing is - you guessed it - water. Here are four reasons why water is the preferred liquid for measuring wetting properties via contact angle: (a) it's a cheap, practically free pure liquid; (b) water has a high surface tension due to the strong hydrogen bonds and thus will produce higher contact angles which are easier to measure and highly repeatable; (c) unlike many solvents, water won't damage most surfaces; (d) since contact angle measurements are sensitive to contamination and since pure deionized water is readily available, it makes for a good choice.
10. Surface roughness requires different models for understanding wetting behavior. For homogenous surfaces, the Wenzel model is used. For surfaces that exhibit heterogeneity relative to chemical composition as well as structure, the Cassie-Baxter model is used.5
11. The highest "natural" contact angle on an homogenous flat surface is around 120°. When the contact angle is much greater - for example, over 150° - then the surface is exhibiting the "Lotus Effect" and is said to be superhydrophobic. These surfaces have microstructures that cause the drop to persist in a Cassie state - that is, the drop is sitting on microstructures and nanostructures (often hierarchical structures).
12. There is no way to measure contact angle that is more fun than using a ramé-hart contact angle goniometer. We've been making lab-quality contact angle tools for over 50 years. We have more experience than any one in the world. And each day we strive to deliver time-proven products while innovating new and exciting solutions for our customers.
1 More on this behavior
September 2010 Newsletter.
|Tools for Measuring Contact Angle|
All ramé-hart instruments measure contact angle. It's in our blood; it's what we do.
The most basic tool we currently offer is our popular Model 190. This is a great choice if contact angle is your primary motive. If you also need to measure the surface energy of solid substrates, consider Model 200 which adds a suite of (6) surface energy tools. If you also need to measure surface tension or need a powerful methods-based experiment design tool for doing dynamic and time-dependant studies, consider Model 250 which includes our DROPimage Advanced software.
Model 260 is a similar configuration but includes our Advanced 3-Axis Stage with fine and coarse vertical adjustment and also sports a higher payload capacity. This allows Model 260 to support the Environmental Chamber p/n 100-07 as well as other accessories.
If you really do a lot of advancing and receding studies, then Model 290 is a good choice. In addition to the Automated Tilting Base, Model 290 also includes the Automated Dispensing System. Model 590 is a similar configuration but features a longer optical bench and the Advanced 3-Axis Stage. Thus, there are more accessories that are available for this model. Model 500 is the same as Model 590 except without the tilting base and dispenser.
That leaves two final choices: Model 400 is optimized for our customers in the semiconductor industry and includes an 8 inch (200mm) rotating wafer support. And Model 790 is our most advanced and feature-laden model that can do everything.
Whatever your requirements may be,
contact us today for a quotation or if you have any questions on
any of our instrument products or accessories.